Epoxy resins have excellent mechanical properties, and therefore, they are used in various industrial applications including coating compositions, adhesives, electric and electronic information materials, and advanced composite materials. More specifically, epoxy resins are frequently used in the fiber-reinforced composite material comprising a reinforcement fiber such as carbon fiber, glass fiber, and aramid fiber and a matrix resin.
Use of a prepreg produced by impregnating an epoxy resin in the carbon fiber substrate is popular in the production of the fiber-reinforced composite material. The prepreg is laminated or preformed, and then heated to cure the epoxy resin and thereby produce a molded article. The property primarily required for a prepreg is that the molded article exhibits good mechanical properties. Recently, however, other properties such as good productivity, namely, high-speed curability are also required. This trend is prominent in industrial applications such as automobile applications where the productivity is particularly needed.
In addition, current prepregs are reactive at room temperature, and freezer installation is generally required for the storage of the prepreg. This requires preliminary arrangement of the freezer installation and thawing before the use. Accordingly, there has been a demand for a prepreg having an improved storage stability which can be stored at room temperature and which have high handling convenience.
With regard to the technology of improving the storage stability, Patent Document 1 and Patent Document 2 disclose a stabilization method by microcapsulation, namely, by coating the surface of the epoxy curing agent by a substance which does not react with the epoxy resin.
Patent Document 3 discloses a method for coating the surface of size-controlled amine particles with a borate ester compound. Patent Document 3 also describes that storage stability and curability can be realized to sufficient degree.
Patent Document 4 discloses a method wherein time between the start of the curing and the reaching of the curing degree to certain level is controlled, and wherein a curing agent having a limited particle size and limited curing temperature is used. Patent Document 4 describes simultaneous realization of storage stability and high-speed curability.